[Freight Guy]

Good point kingairer, but did that runway incursion occur when we were speaking on the interphone with the FA, or while making the announcement? In other words, if you need to stop simply inform ground that you will need to hold your position on the taxiway for a moment to make an announcement to your passengers, heck when they ask why you can inform them that you hear one having a loud discussion with an FA about company/FAA policies and are afraid it might get out of hand and put that crew member at risk to bodily harm if allowed to continue, that way you'll have the Police to explain to your passengers why they can't use their phones. I don't disagree with your reasoning, but there are always many many circumstances that differ from one incident to the next, and if you're truly in a very critical moment that will not allow you to use the PA while moving, then by all means DO stop the airplane and get the situation under control, the way I see it is that if you hear the FA and passenger(s) arguing about the phone issue your attention will be just as diverted from paying attention to what you are doing as it would be if you made an announcement over the PA.
In other words, if you feel you have to stop the aircraft and get the situation under control, do it, if you don't then don't. It's a judgement call based on the situation at hand, if the company policy however does not allow a PA announcement to be made during taxi, and or you are not in a position to do so then stop and do it, but if things in the back are starting to get out of hand you might not want to wait too long before making that call.